Magnet table for rotary screen printing machines

Abstract: Magnet table for rotary screen printing machines comprising a pair of spaced apart parallel girders (11, 11') and a third girder (12) movable therebetween which bears on its top at least a pair of permanent magnets (15, 15'), the upper part of the pair of girders (11, 11') and the interposed room being covered by a buffering cover (10) made of a non-magnetic alloy.

Description: The present invention relates to a magnet table for rotary screen printing machines and in particular a table comprising permanent magnets. It is known that webs of textiles and other soft materials wound on bobbin are printed by means of rotary screen printing machines. The machines of this kind comprise an horizontal drum which supports the printing screen provided with the pattern to be printed. The printing ink is introduced into the printing drum and is pushed through the printing screen by means of a squeegee roll which freely rotates in contact with the internal cylindrical surface of the printing screen supported by the drum during the rotation thereof. The material to be printed is fed beneath the drum by a flexibile conveyor which moves on a magnet table comprising a rigid transverse girder containing electrical magnets. The squeegee roll, made of ferromagnetic metal, is downwardly attracted by the magnet table so that the magnetic attraction causes the squeegee roll to "squeeze" the printing ink through the perforations of the printing screen, with the final result that the printing ink penetrates into the web of the material which is correspondingly printed. The use of an electromagnet in the magnet table for attracting the squeegee roll has the advantage that the attraction can be easily interrupted by simply interrupting the electric power. This operation is quite frequent since it is required any time the squeegee roll is to be replaced or the printing screen to be changed. The use of the electromagnets has however the drawback that the attraction forces are not enough strong to cause the squeegee roll to energically push the ink to penetrate into fabrics of high thickness. In such cases it is possible to increase the forces by using more powerful electromagnets of bigger sizes, but this solution involves room problems. Moreover the electromagnets require a considerable consumption of electric power and are subject to thermal expansion during the operation with consequent negative influence on the printing accuracy. In FR-A-1553987 there is described a magnetic table wherein a girder bearing a set of permanent magnets can be moved parallel to the axis of the squeegee roll in order to modify the magnetic field generated by the magnets. The cover of the magnetic table consists of a plate made of a non-magnetic alloy wherein there is inserted a plurality of discs made of magnetizable material. These discs allow to shunt the poles of the permanent magnets and therefore interrupt their attraction force outside the table. Because of the discontinous structure of its cover, this table has the drawback that the attraction force is not homogeneous along its length and cannot reach very high values. In FR-A-2284448 there is described a magnetic table whose cover is thin and monolithic. However, in this magnetic table the magnets are not movable, so that the attraction force cannot be quickly interrupted. It is therefore a main object of the present invention to provide a magnet table for rotary screen printing machines, which is free of the above mentioned inconveniences. The said object is achieved with a magnet table comprising a pair of spaced apart parallel girders and a third girder placed therebetween which bears on its top at least a pair of permanent magnets, characterized in that said third girder is vertically movable downwards from the upper part of the pair of girders which are covered together with the interposed room by a buffering cover made of a non-magnetic alloy. The features and the advantages of the magnet table according to the present invention will appear evident to those skilled in the art from the following detailed description of an embodiment thereof with reference to the attached drawings wherein: FIGURE 1 shows a schematic and sectional side view of a magnet table according to the present invention, in operating condition; and FIGURE 2 shows a schematic and sectional front view of the same magnet table in operating condition. With reference to Fig. 1, the printing drum 1 rotates in the direction indicated by arrow 2 while printing ink 3 is introduced inside the printing drum and collects on the internal surface of the printing screen 1 in contact with the squeegee roll 4 which is caused to rotate in the direction of the arrow 5 by the rotation of the printing drum 1. The fabric 6, or other similar material to be printed, is fed by the printing blanket 7 which acts as a conveyor and moves in the direction indicated by arrow 8. The arrangement heretofore described is exactly as in the prior art. The novel part in Fig. 1 is the magnet table according to the present invention which will be now described in greater detail. The table comprises a pair of parallel transverse girders 11 and 11' and a third girder 12 movable therebetween which bears on its top at least a pair of permanent magnets 15 and 15', the upper part of girders 11 and 11' being covered by a buffering cover 10. The girders 11, 11' and 12 are made of the same material used for the girders of the tables of the prior art, but their thickness is obviously about one third. The buffering cover 10 consists of a thin sheet of a non-magnetic alloy properly bent as a box in order to cover the upper part of supporting traverse girders 11 and 11'. The permanent magnets 15 and 15' are anchored on the top of the girder 12 in a known way by the bent plate 12A. The number of the magnets 15, 15' depends on their lengths and the length of the squeegee roll 4. The total length of the magnets must at least correspond to the length of the squeegee roll. In order to ensure a perfect alignment of the squeegee roll with respect to its axis, the magnets and their supports have preferably a width in the range of 16-25 mm. The girder 12 is driven by known means (not shown) and can vertically move as indicated by the arrows 13 or 14. With the movement according to arrow 13 the girder 12 brings the permanent magnets in a position very close to the squeegee roll 4 which is in this way attracted by the magnetic field generated by the permanent magnets 15 and 15'. When attracted by the magnetic field the squeegee roll 4 "squeezes" the printing ink through the perforations of the printing screen 1 supported by the printing drum 1 and causes it to penetrate into the fabric 6 to be printed. When the printing operation has to be interrupted, for example when the squeegee roll is to be replaced, the girder 12 is moved in the direction of the arrow 14 to reach a distance from the squeegee roll 4 so that this is no more attracted by the magnetic field generated by permanent magnets 15 and 15'. With reference to Fig. 2, permanent magnets 15 are anchored on the top of the girder 12 close to each other in order to uniformly distribute their attractive force along the whole lenght of the squeegee roll 4. The permanent magnets 15 are selected among those of high intensity such as the ones made of a Nd-Fe-B alloy. In order to ensure a constant pressure of the squeegee roll along the width of the fabric, the distance between the magnets is preferably in the range of 30-40 mm according to the diameter of the squeegee roll and consequently to its stiffness. An important feature of the device according to the present invention is the buffering cover 10. This cover prevents that, when the girder 12 is retracted according to arrow 14 or moving away magnet 15 from squeegee roll 4, the printing drum 1 pushes the printing blanket 7 and the interposed fabric 6 into the room left free by the retraction of the magnets 15. This downwardly movement of printing drum 1, fabric 6 and printing blanket 7 would be detrimental to the functionality of the machine and to the quality of the printing results. The buffering cover 10 is properly made of a non-magnetic alloy so that it will not be attracted by the magnetic field generated by the permanent magnets 15. The metal sheet used for preparing the buffering cover 10 is preferably very thin in order not to reduce in a significant way the attracting action of the underlying permanent magnets 15. However, the thickness of the said metal sheet must be properly selected in order to prevent that it might be deflected by the weight of the printing drum 1 and squeegee roll 4. All these requirements are met for example by a 0.3 mm thick metal plate of a special copper alloy comprising 1.8 to 2% beryllium, more than 0.2% cobalt, about 0.02% nickel and about 0.04% iron. The magnet table according to the present invention has a very simple and compact structure which allows to generate a magnetic field stronger than the ones generated by electromagnets having the same dimensions. The squeegee roll 4 is attracted in a very uniform way along its lenght so that it does not vibrate.

Claims: Magnet table for rotary screen printing machines comprising a pair of spaced apart parallel girders (11, 11') and a third girder (12) placed therebetween which bears on its top at least a pair of permanent magnets (15, 15'), characterized in that said third girder (12) is vertically movable downwards from the upper part of the pair of girders (11, 11') which are covered together with the interposed room by a buffering cover (10) made of a non-magnetic alloy. Magnet table according to claim 1, characterized in that the buffering cover (10) is made of a sheet of a copper alloy comprising beryllium, cobalt, nickel and iron. Magnet table according to claim 1, characterized in that the buffering cover (10) is made of a sheet of a copper alloy comprising 1.8 to 2% beryllium, more than 0.2% cobalt, 0.02% nickel and 0.04% iron. Magnet table according to claim 1 and 2, characterized in that the permanent magnets (15, 15') are of high intensity type made of a Nd-Fe-B alloy.

Inventors: MOLTENI DANILO, MOLTENI, DANILO
Applicants: SGM SPA, SGM S.P.A.
Application year: 1994
Main classification: B41F15
IPC codes: B41F15, D06P5